1. Field of the Invention
This invention relates to a differential pressure forming machine for forming an article in a sheet of thermoplastic material and more particularly to infrared energy sensing apparatus for sensing the temperature of the thermoplastic sheet between the sheet heating station and a differential pressure forming station.
2. Description of the Prior Art and Objects
Differential pressure forming machines have been provided heretofore for forming articles in a thermoplastic sheet of material. Such differential pressure machines are illustrated in U.S. Pat. No. Re. 27,137 granted to Gaylord W. Brown et al on Jun. 8, 1971.
The prior art differential pressure forming machines include a sheet heating oven through which a thermoplastic sheet is passed and heated before it is moved to a forming station. The ovens have included quartz or ceramic heaters having thermocouples in the heater elements which control the heater element temperature.
The sheet temperature is a critical, if not the most critical, variable among the various thermoforming variables, such as vacuum and air pressure, in the forming process. The prior art method of reading temperature of heaters with thermocouples, which were utilized to control the heater element, did not necessarily measure the sheet temperature. The sheet temperature may or may not follow or correlate to the temperature of the heating element depending on the various factors such as the quality and quantity of ambient air around the sheet.
Infrared sensors have been disposed between the heater and the forming station. Unfortunately, the placement of an infrared sensor at a location between the sheet heating oven and the differential pressure forming apparatus required substantial valuable space in the differential forming process. This construction has the disadvantage of providing a relatively large distance between the heating station and the forming station in which the sheet is allowed to cool. If a sheet cools too much between the heater and the former, scrap can result. By narrowing the gap between the heating station and the forming station by apparatus constructed according to the present invention, the scrap is reduced. Accordingly, it is an object of the present invention to provide a differential pressure forming machine having a new and novel sheet temperature sensing apparatus.
It is another object of the present invention to provide apparatus for sensing the temperature of a sheet which will decrease the scrap rate in a differential pressure forming machine.
A further object of the present invention is to provide apparatus for sensing the temperature of a thermoplastic sheet in a differential pressure forming machine including a hollow infrared energy wave guide which is disposed between the sheet heating oven and a differential pressure mold apparatus for guiding infrared energy to a remotely located infrared energy sensing apparatus.
Apparatus such as that disclosed in U.S. Pat. No. 4,933,126 granted to Jack N. Shuman on Jun. 12, 1990, includes an infrared thermometer mounted on a cylindrical sleeve or guide which extends centrally through a heating oven in the midst of the heating elements. The infrared thermometer will be affected by the heat emitted by the heating element as opposed to sensing only the infrared energy emitted by the heated sheet. Accordingly, it is another object of the present invention to provide an infrared energy sensing system which will be less affected by the heat emanating from the oven of a differential pressure forming machine.
A still further object of the present invention to provide apparatus for sensing the temperature of a thermoplastic sheet at a location between a sheet heating station and a differential pressure sheet forming station including remotely located infrared energy sensors and a wave guide for guiding the infrared energy emitted by the sheet to the infrared energy sensors.
Another object of the present invention is to provide apparatus for sensing the temperature of a thermoplastic sheet at a location between the sheet heating station and sheet forming station in a differential pressure forming machine including an infrared energy sensor located adjacent the sheet heating oven and a hollow wave guide tube having a 90.degree. bend therein for receiving and transmitting the infrared energy emitted by the sheet at a location between the oven and the former in a 90.degree. path to the infrared sensors.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.